This invention relates generally to improved plating baths for the electroless deposition of transition metals, which baths include a complexing agent belonging to a certain class of organophosphorus compounds, the electroless deposition complexing agent of this invention being especially suitable for use in formulating a variety of baths having a wide range of operating pH values and proceeding by one of a variety of reduction reaction mechanisms. Especially advantageous are those formulations that have a very high operating pH and which avoid the production of noxious ammonia fumes.
Electroless plating techniques have been available for transition metal plating within electroless plating baths, which are well known to be baths wherein the plating is conducted without the application of an external electrical current. Conventional bath additives include agents for complexing the metal ions that are maintained within a particular bath. Some known complexing agents include ethylenediamine tetraacetic acid and its salts, potassium pyrophosphate, polyamines, Rochelle salt, citric acid, and alkali salts of citric acid. Such complexing agents are known in general to form highly soluble and conductive complexes with metal ions present within electroless plating baths, but typically they do not allow for an exceptionally wide range of operating pH values, including both acidic and alkaline operating conditions. Usually, when it is desirable or necessary to operate an electroless plating bath at an especially high pH, often approaching or at 14, even if a conventional complexing agent is included in the bath, it is typically necessary to add ammonium ions thereto, which introduces hazards including the formation of noxious ammonia fumes as the bath is operated and the ammonium ions themselves when the bath waters are disposed of as wastewater.
It has now been discovered that electroless deposition of transition metals can be readily and easily achieved within a wide range of operating pH values, including both acidic and highly alkaline values, when such baths include therewithin an organophosphorus compound falling within a certain class. These organophosphorus compounds can be characterized generally as phosphonates that exhibit complexing properties with respect to transition metal ions within transition metal electroless plating baths. By the employment of these organophosphorus compounds within transition metal electroless plating baths, the baths may be adjusted to operate at a pH value suitable for the particular substrate being plated so as to provide transition metal plating baths having greater flexibility than heretofore typically realized.
Additional desirable results include the ability to run a highly alkaline bath without having to add ammonium ions as well as the ability to operate an electroless transition metal bath, particularly an electroless copper deposition bath, at an acidic operating pH in order to take advantage of features usually associated with acidic baths when compared with alkaline baths, such as the ease with which the bath is buffered, increased hardness of the deposits, and enhanced control of the stability of the baths. Baths not including complexing agents in accordance with this invention, particularly electroless nickel plating baths, often are not stable to the addition of potassium or sodium as potassium hydroxide or sodium hydroxide, usually eliminating these compounds from the group of bases suitable for raising the pH of such baths to extremely high operating levels, thereby typically reducing the group of suitable bases to only ammonium hydroxide. Electroless transition metal plating baths having complexing agents in accordance with this invention are stable to the addition of either sodium or potassium as the hydroxide thereof.
These results are basically achieved in accordance with the present invention through the utilization of certain organophosphorus compounds within electroless transition metal plating baths which can be formulated with transition metals that plate autocatalytically, such as nickel, cobalt, iron, platinum, paladium, gold and silver, or with those transition metals that do not plate autocatalytically, such as copper, which generally plate only by substitution or displacement and require a substrate different from that of the metal being deposited.
Accordingly, an object of this invention is to provide an improved electroless plating bath and method which enable the electroless plating of transitional metal over a wide pH range.
Another object of this invention is to incorporate an organophosphorus complexing agent into transition metal electroless plating baths, which baths can be formulated to operate at a low pH, a high pH, or a pH therebetween and which baths can operate effectively at high pH through the use of bases other than those that generate noxious ammonia fumes or introduce ammonium ions into wastewaters.
Another object of the present invention is to provide and use an electroless bath for plating transition metals such as copper, nickel and cobalt, which baths include an organophosphorus compound of a certain class.
Another object of this invention is to use phosphonates belonging to a certain class of compounds as a complexing agent for an electroless transition metal plating or deposition bath.